When a mechanical part required to have a high surface hardness, such as a gear, is produced using a steel as a material thereof, carburization and high-frequency hardening are typical as means for surface hardening. Carburization is an operation for increasing a carbon concentration of a part surface. As the result of the carburization, high mechanical strength may be obtained. However, owing to the hardening following the carburization, the part is rendered a martensite phase to the inside of the part; accordingly, there is a problem that the residual strain is large. On the other hand, high-frequency hardening is an operation that hardens only a surface a part without changing the internal structure of the part; accordingly, there is an advantage that the strain is small. However, from the viewpoint of productivity, since there is a limit on an attainable surface carbon concentration, high mechanical strength may not be expected.
A surface treatment where the carburization and the high-frequency hardening are combined to realize features of the both has been attempted. As one of these, a technology where the carburizing and hardening are applied under a specific carbon potential to obtain a carburized product in which a difference of a surface carbon concentration and a core carbon concentration is rendered a definite value or more, and the carburized product is then subjected to a high-frequency hardening to austenitize 0.3 to 1.5 times a total hardened layer depth at the time of carburization has been disclosed in JP-A-64-36779. According to this document, it is described that an old austenite grain size that affects on the fatigue strength of the carburized product may be miniaturized to No. 10 or more in accordance with the JIS grain size number.
There is another proposal where a case-hardened steel having a specific alloy composition is, after carburizing and hardening, tempered at a temperature in the range of 400 to 600° C., followed by applying high-frequency hardening to recover the hardness of a carburized layer softened by the tempering (see, JP-A-5-255733). According to this document, it is described that a product obtained by this method sufficiently secures various characteristics usually required for the carburized product such as the tensile strength, impact strength, carburized layer hardness and like, as well as excellent delayed fracture resistance.
In order to improve the fatigue strength and the impact strength of a part, it is necessary to miniaturize the grain size. In this regard, there has been proposed a heat treatment process where, after the carbonitriding and hardening, a second hardening step where steel is heated to an austenite region and then hardened is carried out, in order to realize miniaturization of grain size in surface hardened parts (see, JP-A-10-18020). According to this method, owing to the carbonitriding and hardening, a steel containing carbon and nitrogen is transformed from an austenite phase to a martensite phase and the grain size is subsequently miniaturized according to the second hardening.
The inventors intended gradually cooling from a carburization step without applying the hardening and hardening at s subsequent high-frequency hardening, to thereby heighten the surface hardness while averting generation of strain caused by the carburizing and hardening. When JIS steel species such as SCR420 and SCM420, that have been used in producing a carburized product, are used as a material, desired mechanical strength may not be obtained. As a result of investigating the reason for this, it was found that carbide generated at the carburization remains without dissolving in a matrix in the step of high-frequency hardening and the carbide in the carburized product becomes a starting point to forward the fracture. In the high-frequency hardening, since a heating time is short, the carbide does not have enough time to dissolve in to the matrix. In order to avert this, it is necessary to lower a carbon concentration. However, since there is a proportional relationship between the surface carbon concentration and the mechanical strength, when the carbon concentration is lowered, the mechanical strength becomes deteriorated.
As a means for carburizing, in addition to a conventional gas carburization, recently, a vacuum carburization has been frequently employed. The vacuum carburization has, for example, the following advantages in comparison with the gas carburization.
1) Since a grain boundary oxidation at the carburization may be averted, high mechanical strength tends to be readily obtained.
2) The carburization can be rapidly conducted due to the easiness of a high temperature operation.
3) The running cost is inexpensive.
In view of these advantages, the vacuum carburization is used to produce various kinds of gears and shafts. However, there is a problem that the difference of the carbon concentrations depending on positions of a part is large. In particular, a portion having an edge shape tends to be high in the carbon concentration, and therefore, an amount of residual carbon as well is abundant there, whereby it is more difficult to combine the vacuum carburization with the high-frequency hardening. In this connection, the inventors considered a mechanism where an alloy composition in which carbide is difficult to generate is selected, whereby the carbide is not necessarily expected to dissolve during high-frequency hardening and established an alloy composition of carburized steel that realizes the mechanism.